With the development of the Light Emitting Diode (LED) for use in the lighting industry, the opportunity for energy savings continues to be significant for individuals and society as a whole. A major hurdle impeding the realization of energy savings, however, is the cost of installation or manufacturing. In particular, many of the new technological improvements which greatly improve the luminous efficacy of white LEDs come with disadvantages in the manufacturing process.
In the manufacturing of a common white light LED, a phosphor powder blend is normally mixed with an encapsulant and deposited as a layer onto the surface of an emitter junction. Many emitter junctions are narrow wavelength band blue or near ultraviolet (UV) diodes, with a Full Width at Half Maximum (FWHM) of only 40 nm. The phosphor blend is created to absorb the blue or UV wavelengths and reemit broadband green to red wavelengths. The ratio of blue absorption to secondary green to red emissions determines the color temperature of the emitted white light. This is a straightforward process and as such any improvements to increase the luminosity output add complications to this process which can increase the manufacturing cost.
Failures that plague the above approach include heat degradation of the phosphor due to its proximity to the emitter junctions. The emitter junctions run at relatively high temperatures (>70 deg C.) against an ambient background (25 deg C.). The encapsulant degrades with higher temperatures, often resulting in discoloration and inefficient transmitters in the blue or the visible wavelength region. Additionally, the encapsulant acts as a thermal blanket on top of the emitter junctions causing a further loss of efficiency. Close proximity of the phosphor layer to the emitter junctions also creates an additional loss of white luminosity, as efficient emitters by definition (thermo-dynamic blackbodies) must also be efficient absorbers, thus white light generated near the emitters is lost energy.
There exists another limitation to the above approach: the relatively small area of emission. This creates an increase in luminosity but spreads the light output over a small cone angle, thus creating a very high brightness. Such brightness levels can pose a threat to the human vision system and can contribute to migraines, seizures, and temporary washout of the human eye. Yet another limitation to the above approach is that it is not directly scalable. It is not an easy task to increase luminosity by adding more emitter junctions and thicker layers of phosphor. Such endeavors increase the heat load in a non-linear way while adding more of the “blanket” effect from the phosphor encapsulant.